The present invention relates to an improved device for providing the locking of a cable as it passes from one side to the other side of the wall of a tube.
The device according to the invention can be particularly utilized in a side-entry or a side-window sub set on a column taken down into a well such as a well bore or an oil-field development well.
Side-entry subs are well-known in the oil sphere. They are used for the passing of electric carrying cables linking a surface installation with a servicing tool at the bottom of a well bore such as a logging tool. Examples of such subs are for example described in French Patents 2,522,059 or 2,617,232. Such subs are used for example for carrying out loggings in horizontal wells or wells that are highly inclined with respect to the vertical. An appropriate sonde is fastened onto the end of a tubular element which is introduced into the well to be logged. By successively adding string sections, the column is extended until the sonde is brought to a given depth. The column which is constituted thereby is then added a side-entry sub across which is passed an electric carrying cable fitted with a socket connector that can be plugged in a liquid medium topped by a load bar. Through fluid pumping, the socket connector is pushed onto the bottom of the string where it plugs into a multicontact plug that is placed to this effect above the sonde. The cable is then locked in the side-entry sub. The column is then completed by adding complementary string sections, with the cable outside the string, until the sonde in the lower end of the column reaches the inclined area where the measurings are to be carried out. This corresponds to a lowering depth of several hundreds to several thousands of meters. After connecting the cable to a surface control and recording laboratory, the logging measurings are generally achieved by taking the sonde and the associated column up along the area to be explored until the side-entry sub is brought back to the surface.
The locking of the cable in this sub is a practical necessity. Although the cable is rather rigid, its extension under the effect of the tractive stresses is relatively considerable. The extension may reach several meters and even about ten meters when the unwound length is several kilometers. It is therefore not possible to refer to the length of cable unwound from the storage reel in order to know with precision the working depth of the sonde. The total length of the column introduced into the well which is, for its part, much more rigid, is preferably measured and the cable is preferably displaced with the column by locking one in relation to the other at the level of the side-entry sub. The cable is taken up at the same time and at the same velocity as the rods while exerting on it a rather low (1,000 daN for example) constant mechanical stress so that its extension on several hundreds of meters is negligible. A precise measurement of the depth where the loggings are carried out is obtained thereby.
In practice, this locking is the source of certain difficulties, essentially because of the cable manufacturing procedure. It is spiral-shaped and made with twisted prestressed strands. The taking down of the sub where it is locked in translation and rotation, over several hundreds of meters, causes a stress relief of the coils of the cable which turns round on itself by 10 to 20 revolutions over a distance of 500 meters for example. When the sub is taken up, the part of the cable between the latter one and the reel becomes twisted in the opposite direction and progressively recovers its initial stress whereas the length on which the twisting occurs constantly decreases. Over the last meters, the torsional stress is often such that it damages the cable. The recurrence of these alternate stress reliefs and stresses eventually spoils it. This tendancy can especially be noticed when the cable that is used is new.